12 across
Astronomer’s
western
partner,
one
with
a
sign
(7)
Astronomer’s western partner, one with a sign (7)
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The definition is at the beginning of the clue.
How the clue works
Right, so Astronomer's is your definition, pointing to Galileo himself. Then it builds up piece by piece: GAL for a western partner (a girl), I for "one", and LEO for the zodiac sign — stick those together and there's your astronomer.